1. Field of the Invention
The invention relates to a hydrogen production method, a hydrogen production system used to implement the hydrogen production method, and a fuel cell system incorporating the hydrogen production system.
2. Description of the Related Art
In recent years, various researches and studies have been widely conducted on fuel cell systems as one of key technologies to solving the global warming issue. A fuel cell is constituted of an electrolyte layer and a pair of electrodes and causes electrochemical reactions. The electrochemical reactions at each fuel cell generate electric power, and this electric power is output. Among various fuel cell systems, household cogeneration systems and solid-polymer type fuel cell systems typically used for automobiles, etc., (will be referred to as “PEFCs”) use hydrogen-containing gas and oxygen-containing gas. Therefore, in order to put such PEFCs to practical use, reliable hydrogen production technologies and hydrogen storage technologies are essential.
The hydrogen storage technologies that have been proposed so far include compressing hydrogen gas and then storing it in a high-pressure hydrogen tank, storing liquid hydrogen in a liquid hydrogen tank, and storing a hydrogen adsorption material adsorbing hydrogen (will be referred to as “hydrogen production material” where necessary) in a tank. However, in the case where a high-pressure hydrogen tank is used, a large volume of the high-pressure hydrogen tank makes it difficult to provide a compact system, and it also requires high pressurization energy to increase the pressure of hydrogen up to a desired high level, which is not efficient. In the case where a liquid hydrogen tank is used, there are some problems. For example, because it is necessary to continuously cool hydrogen such that its temperature remains at an extremely low temperature (−253° C. or lower), a large amount of energy tends to be consumed to keep hydrogen stored properly, and also it is difficult to avoid so-called “boil-off”, that is, vaporization of liquid hydrogen due to the heat entering the liquid hydrogen tank from the outside. PTFCs in that a small amount of energy tends to be consumed to keep hydrogen stored properly and that is better adopted to avoid “boil-off”, are watched.
Published Japanese Translation of PCT application No. 2002-526658 (JP-A-2002-526658) describes a technology related to hydrogen adsorption materials. This publication proposes a lithium group hydrogen compound having a high hydrogen capacity and capable of increasing the amount of hydrogen irreversibly at a high rate. Further, Japanese Patent Application Publication No. 2005-154232 (JP-A-2005-15422) describes a technology related to hydrogen storage materials. The hydrogen storage material described in this publication is constituted of hydrogen compounds of metal and ammonia, and hydrogen is produced through the reaction between the hydrogen compounds of metal and the ammonia. Further, Japanese Patent Application Publication No. 2006-182598 (JP-A-2006-182598) describes a technology related to hydrogen production. This publication proposes a hydrogen production system that reuses by-products by reacting them with non-separated hydrogen to produce organic hydrides.
It is considered that the use of the hydrogen storage material described in JP-A-2002-526658 makes it possible to produce a large amount of hydrogen in a small mass. However, the temperate range for the hydrogen producing reactions of this hydrogen storage material is approx. 200 to 300° C., and therefore the heat for causing the reactions needs to be supplied externally. For this reason, if this hydrogen storage material is used in a hydrogen production system, the overall power generation efficiency of said system tends to be low, which is undesirable. Meanwhile, it is considered that the use of the hydrogen storage material described in JP-A-2005-154232 makes it possible to produce a large amount of hydrogen. However, because the ammonia used for hydrogen production needs to be fully stored in the hydrogen production system in advance, a system for controlling the odor of ammonia needs to be additionally incorporated in the hydrogen production system, and therefore this hydrogen storage material is not suitable for the use in vehicles. Further, according to the technology described in JP-A-2006-182598, hydrogen production can be continued cyclically using the produced organic hydrides for the hydrogen production. However, the amount of hydrogen produced per unit mass (will be referred to as “hydrogen capacity”) of organic hydrides, which are typified by decalin, etc., is relatively small, and the hydrogen producing reactions of organic hydrides occur in a temperature range of approx. 200 to 350° C., which is undesirable as in the case of JP-A-2002-526658.
In order to realize a sustainable hydrogen society, it is required to produce hydrogen from a small amount of energy, to reuse or recycle hydrogen production materials, to avoid newly producing waste upon hydrogen production and upon reusing or recycling of hydrogen production materials, and to use hydrogen production materials having a large hydrogen capacity. However, it is considered that none of the hydrogen production materials proposed so far satisfies all these requirements. That is, none of the hydrogen production materials proposed so far satisfies these general requirements and is suitable for the use in small vehicles.